Our current knowledge of the host-parasite relationship is reviewed as part of a series entitled 'Papers of yesterday and today'. A retrospective view is taken of a paper written in 1898 by Professor H. J. Hamburger, entitled 'The Current Point of View of the Theory of Natural Immunity', which is also published in a slightly abridged version in this issue of Tijdschrift voor Diergeneeskunde. A review of the history of developments in research in the field of infectious diseases since 1898 is followed by a discussion of two phenomena, colonisation resistance and immune adherence. So-called non-specific as well as specific (= immunological) defense mechanisms are then discussed. Finally, our current knowledge of the 'Biological Response Modifiers' is considered particular attention being paid to possible uses of recombinant DNA interferons and interleukins in human and veterinary medicine.